Since the 1970's, the ever increasing cost of energy derived from fossil fuels has become of critical concern throughout the world. One consequence of the possible depletion of fossil fuel reserves and the increasing cost of energy produced therefrom has been heightened interest in the applications of solar energy, particularly as a means for space and hot water heating for buildings.
A myriad of solar heating methods and systems of both the "active" and "passive" types has heretofore been proposed. Of particular current interest are the large variety of passive solar heating designs in which the collection, storage and distribution of solar energy in a building or the like is effected by natural means, with but minimum use of relatively complex (and expensive) solar panels, pumps, fans and other auxiliary mechanical systems. Illustrative of some of the many passive solar heating systems described in the prior patent literature, for example, are those disclosed in Thomason U.S. Pat. Nos. 3,254,703 and 3,412,728; Hay U.S. Pat. No. 3,563,305; Nilsson U.S. Pat. No. 4,006,856; and Groth U.S. Pat. No. 4,029,258.
Prior solar heating systems provide only for the short-term storage of solar energy for no more than a few days at a time. In effect, such systems require fast, efficient energy recovery to facilitate use substantially concurrently with solar energy collection or, at the most, after storage of no more than a few days as, for instance, during periods of cloudy, rainy or snowy weather. Such systems are inadequate in many regions which experience long periods of overcast climatic conditions. Moreover, by their very nature they require the additional provision of complex and expensive auxiliary equipment, e.g., conventional heating and/or cooling systems, as supplemental energy sources. Accordingly, prior art solar heating systems have found only limited application to date.
The prior art systems all employ some form of short term energy storage such as a collection of stones or a pool of water. The use of the earth as a long term thermal storage reservoir has also been proposed, for example, in Werner U.S. Pat. Nos. 4,024,910 and 4,139,321; Wade U.S. Pat. No. 4,128,204; and Yuan U.S. Pat. No. 4,138,995. However, these proposals require the use of relatively complex ducts, channels and/or piping arrangements, or the provision of pumps for the active circulation, i.e. pumping, of heat transfer fluids to effect thermal energy storage within the underground reservoirs provided thereby. Moreover, the circulating fluid is also circulated to the thermal load in many of these instances whereby to reduce the availability for storage of thermal energy in the fluid. The Yuan device comprehends the use of plural heat transfer fluids, e.g., water ammonia, acetone, fluorocarbon refrigerants, alcohols or liquid metals, and plural, active or pumped systems for effecting heat exchange with the underground thermal storage zone thereof. Such prior art teachings thus necessitate the use of relatively complex "plumbing", and/or require "active" mechanical systems for thermal energy storage and distribution.